An artifact is something you can point to that represents a moment of your professional growth as an educator.
For example, when I was a student teacher (many years ago), I was asked to create a quiz for the class that I was serving as lead instructor. I developed a quiz that I believed represented tasks the class should have been able to complete given what I had taught the previous week. After I gave the assessment and saw the poor scores, my clinical instructor knowingly shared with me how the assessment I designed did not allow for differentiation in performance. It was more a challenge problem that frustrated most of the students, thus not allowing for an accurate measure of what the students knew about the content that I taught. It was a bad test! BUT, most importantly, that moment of failure represented an important moment in my development as a teacher in what makes a good assessment of learning. I wanted to know what they knew, not frustrate them so I didn't get the information that I sought after.
That test is an artifact of my professional growth as an educator. But if I put the test in front of you it wouldn't mean anything without the context and reflection that I included above. So an artifact is something you can point to that represents something bigger.
So in selecting an artifact consider the questions:
How have I developed as a professional during this semester? class? field experience?
What artifact would represent that moment of professional growth?
Remember, the only one who can tell you how you grew as a professional is YOU. You identify those moments, then (if needed) seek help identifying the artifact that represents that moment.